1. Field of the Invention
The present invention relates to sublimation type thermal printers wherein dye is transferred to recording paper through sublimation and diffusion process caused by the heat from a thermal head.
2. Description of the Prior Art
A typical thermal printer has a structure such as that shown in FIG. 3. More specifically, the printer comprises a platen drum 1, a donor cartridge 2 and a thermal head 3. In forming an image, a sheet of recording paper R which is placed on the platen drum 1 is rotated, while a dye donor film I, which is accommodated in the donor cartridge 2, is fed in such a manner as to bring the recording paper R into contact with the platen drum 1, and the thermal head 3 presses against the donor film and the recording paper R.
The thermal head 3 has a large number of heating elements or resistors which are turned on and off in accordance with information for image formation in order to heat the dye donor film I and thus to transfer dye to the recording paper R through a sublimation and diffusion process, thereby forming a desired image on the paper R.
When a full-color image is to be formed, the platen drum 1 is rotated three full turns and each of the three different dyes, that is, yellow, cyan and magenta, is transferred during each revolution of the platen drum 1, thus forming a full-color image. For a more complete description of a thermal printer, see commonly assigned U.S. Pat. No. 4,691,211 to Brownstein.
The above-described conventional printer suffers, however, from the following problems. Dyes which have been transferred during the first and second turns of the platen drum 1 to form a full-color image are passed under the thermal head 3 during the respectively subsequent second and third turns of the drum 1 and are therefore subjected to the heat accumulated in the thermal head 3, thus achieving stabilized fixing. However,the dye which is transferred during the third turn of the platen drum 1 is subjected to only one heating by the thermal head 3 and is therefore not sufficiently heated to be able to penetrate into the recording paper R to the full, so that it is impossible to achieve stabilized fixing.
In order to overcome this problems, it is conventional practice to pass the recording paper from the printer through a heater (fuser) so as to stabilize the transferred dyes. However, this conventional practice necessitates preparation of two separate apparatus, i.e., a printer and a fuser, which increases the cost, requires more space, and takes more processing time.
There is another problem with the sublimation type thermal printer. The thermal head should preferably be maintained at a predetermined temperature so that it is possible to obtain stabilized density when printing is started. If the temperature of the thermal head is not maintained above a certain level, the temperature of the head, which is relatively low when use of the head is started, becomes relatively high after it has been used for a certain period of time due to the accumulation of heat, which results in unacceptable density variations in the resulting print.